When trades are automatically canceled it means one or more creatures have been traded with someone else meanwhile from either side.
For example. I send you a trade, I send a destrinos for a flamiris.
I send someone else a trade including my same destrinos for someone esle's stoufix.
If the second person accepts the trade first, then the trade with you will be automatically canceled.
I hope this makes sense, I'm not the best at explanations